I think you are the first to try RichFaces with JSFUnit. I plan to look at it more closely after beta unless someone else wants to step up and get RichFaces support working sooner.
Since RichFaces is based on ajax4jsf, there might not be that much work to do. But I'm just not sure because I haven't tried it.
If I get some time I'll try to look at this a bit closer. Does <rich:suggestionbox> support having <a4j:support> nested inside?
ComponentIDNotFoundException is telling you that no component ID ending in "a4jProductSuggestion" was found in the HTML. If you look at the HTML and see what was rendered you can tell if this is really the case.
i will have a look and I give you a feed back soon :)